1. Field of the Invention
The present invention relates to a color filter substrate and to a liquid crystal display device provided with the color filter substrate. In particular, the present invention relates to a color filter substrate suitable for driving liquid crystal by means of oblique electric field caused by applying voltage between a transparent conductive film formed for the color filter substrate and first and second electrodes formed for an array substrate, and a liquid crystal display device provided with the color filter substrate.
2. Description of the Related Art
In recent years, thin display devices such as a liquid crystal display device are increasingly demanded to enhance the picture quality and power-saving thereof and to reduce the manufacturing cost thereof. In the case of the color filter to be employed in such display devices, it is demanded to exhibit sufficient color purity, high contrast, and flatness in order to obtain high image quality.
In liquid crystal display device having high image quality, various alignment systems and driving systems of liquid crystal are proposed. Liquid crystal cells employing these systems includes VA (Vertically Aligned), HAN (Hybrid Aligned Nematic), TN (Twisted Nematic), OCB (Optically Compensated Bend), and CPA (Continuous Pinwheel Alighnment). Display device of a wide viewing angle and high speed response are realized by these liquid crystal cells.
In liquid crystal display devices of VA system which is easily applicable to high speed response in a wide viewing angle and in which liquid crystal molecules are aligned vertical to a surface of a substrate made of glass, and of HAN system which is effective in wide viewing angle, it is demanded to obtain further high level flatness in regard to color filter (uniformity of film thickness and evenness of the surface of color filter) and electric properties such as dielectric constant. In such liquid crystal display devices having high image quality, it is a problem to decrease a film thickness of liquid crystal cell (liquid crystal layer) in order to suppress coloring of image in oblique viewing direction. In the liquid crystal display devices of VA system, various improved modes have been developed in order to solve such problem, the modes including multi-domain vertically alignment (MVA) mode, patterned vertically alignment (PVA) mode, vertically alignment electrically controlled birefringence (VAECB) mode, vertically alignment hybrid-aligned nematic (VAHAN) mode, and vertically alignment twisted nematic (VATN) mode. In the liquid crystal display devices of vertical electric field system such as VA system in which driving voltage is applied in thickness direction of liquid crystal layer, it is demanded to obtain higher response of liquid crystal, wider viewing angle, and higher permeability. MVA is a technique to attain wide viewing angle. In this technique, a plurality of structures named as ribs or slits for restricting alignment of liquid crystal molecules arranged, and domains of liquid crystal in a plurality of alignment directions are formed between the ribs in order to solve the problem of unstable vertical alignment of liquid crystal molecules on application of voltage for driving liquid crystal (a tilt direction of liquid crystal molecules on application of voltage, which are vertically aligned at initial stage, is undeterminate). JP-A 2005-167243 disclosed a technique of forming domains of liquid crystal using first and second structures for restricting alignment (ribs).
When liquid crystal of negative dielectric anisotropy is employed, liquid crystal molecules between ribs made of resin tilts in a direction perpendicular to the ribs as viewed from the front on application of voltage, and are aligned parallel to the surface of the substrate. In this case, tilt direction of liquid crystal molecules positioned at a center of a region between the ribs is indeterminate irrespective of application of voltage, and these liquid crystal molecules act in spraying alignment or bending alignment. These alignment turbulence bring about rough or uneven display. In the case of MVA system, it is difficult to minutely control the tilt amount of liquid crystal molecules by driving voltage. For that reason, a half gray scale display has a difficult point.
In order to solve the problem, the technique of controlling vertical aligned liquid crystal molecules by means of oblique electric field caused by applying voltage between a transparent electrode on the color filter substrate side (transparent electrode or third electrode) and first and second electrodes on the array substrate side is disclosed in Japanese Patent Nos. 2859093 and 4459338. The technique disclosed in Japanese Patent No. 2859093 employs liquid crystal of negative dielectric anisotropy, and the technique disclosed in Japanese Patent No. 4459338 employs liquid crystal of positive dielectric anisotropy.
The technique of controlling vertical aligned liquid crystal molecules by means of oblique electric field caused by applying voltage between a transparent electrode and first and second electrodes as described in Japanese Patent Nos. 2859093 and 4459338 is very effective. It is possible to set a tilt direction of liquid crystal molecules by oblique electric field. Since a tilt amount of liquid crystal molecules can be easily control, it is effective in a halfgray scale display.
Even these techniques are, however, insufficient in preventive measures against disclination of liquid crystal. “Disclination” is a phenomenon in which regions having different transmittances are generated in a pixel (minimum unit in display) by undesirous alignment disorder.
In the technique disclosed in Japanese Patent No. 2859093, an alignment controlling window in which a transparent conductive film is not formed is arranged in the center of the portion of the opposing electrode corresponding to a pixel in order to fix the disclination in the center of the pixel. The document, however, dose not disclose the preventive measures against disclination around the pixel. Though the document discloses the fixing of the disclination in the center of the pixel, does not disclose the measures for minimizing the disclination and improving the response property of liquid crystal.
The technique disclosed in Japanese Patent No. 4459338 is a preferable technique since a dielectric layer formed on a transparent conductive film (transparent electrode) promotes effects of oblique electric field. The technique disclosed in Japanese Patent No. 4459338, however, has a problem of lowering of transmittance or aperture ratio since vertically aligned liquid crystal molecules are remained in the center of the pixel and around the pixel even after application of voltage as shown in FIG. 7 of Japanese Patent No. 4459338. Where liquid crystal of a positive dielectric anisotropy is employed (Japanese Patent No. 4459338 does not disclose liquid crystal of a negative dielectric anisotropy in the description and Examples.), it is difficult to raise transmittance owing to disclination in the center of the pixel. For that reason, it is difficult to employ the technique disclosed in Japanese Patent No. 4459338 in transflective liquid crystal display devices.
Generally, liquid crystal display devices of VA system or TN system have a fundamental structure in which liquid crystal is sandwiched between a color filter substrate having a common electrode and an array substrate having a plurality of pixel electrodes for driving liquid crystal (for example, transparent electrodes having a combteeth shaped pattern and connected to TFT elements). In this structure, liquid crystal is driven by applying voltage between the common electrode on the color filter substrate and the pixel electrode formed on the array substrate side. The pixel electrode and the common electrode are formed of a conductive metal thin film made of indium tin oxide (ITO), indium zinc oxide (IZO), or indium gallium zinc oxide (IGZO).
Color filter structure, in which blue, green, and red pixels are formed above a transparent conductive film, is disclosed in FIG. 2 of JP-A 5-26161. The technique of forming a color filter on a transparent electrode (transparent conductive film) is disclosed in the above-described Japanese Patent No. 4459338 (for example, in FIGS. 7 and 9), though it employs a plurality of stripe electrodes and liquid crystal of a positive dielectric anisotropy.
As a technique of improving a brightness and luminosity in order to obtain a dynamic display of a higher picture quality and to extend a range of chromaticity, a technique performing four-color display in which a yellow pixel or white pixel is added to a red pixel, green pixel, and blue pixel.
These techniques, however, necessitate another pixel such as a yellow pixel and white pixel together with a red pixel, green pixel, and blue pixel, and a further active element for driving the other pixel or color filter layer for forming a color filter, and thus bring about high cost owing to increase in number of manufacturing steps. Further, these techniques necessitate suppression or inactivation of yellow or white display in the gradation display range in which yellow or white display having high brightness is not necessary, and do not bring about effective rise of brightness. Furthermore, these techniques necessitate control of color temperature of a backlight and adjustment of pixel areas of different colors for obtaining a white balance. In addition, there is a problem that yellowish tone is strengthened in a reflective display. For example, a special blue filter shown in Jpn. Pat. Appln. KOKAI Publication No. 2005-352451 is necessary to suppress the yellowish tone.